llvm: Output stderr when generating libs/flags/etc fails
llvm: Output stderr when generating libs/flags/etc fails

f.ex when you don't have the llvm-static package installed, the error
message when generating libs is cryptic and uninformative since we
discard stderr.

jeandet/meson,thiblahute/meson,MathieuDuponchelle/meson,becm/meson,MathieuDuponchelle/meson,thiblahute/meson,ernestask/meson,jpakkane/meson,pexip/meson,jpakkane/meson,MathieuDuponchelle/meson,mesonbuild/meson,becm/meson,jeandet/meson,pexip/meson,jpakkane/meson,ernestask/meson,QuLogic/meson,becm/meson,jpakkane/meson,mesonbuild/meson,becm/meson,pexip/meson,pexip/meson,trhd/meson,QuLogic/meson,QuLogic/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,ernestask/meson,pexip/meson,MathieuDuponchelle/meson,pexip/meson,QuLogic/meson,ernestask/meson,trhd/meson,thiblahute/meson,becm/meson,jpakkane/meson,MathieuDuponchelle/meson,ernestask/meson,jeandet/meson,jpakkane/meson,becm/meson,ernestask/meson,becm/meson,jeandet/meson,trhd/meson,mesonbuild/meson,thiblahute/meson,QuLogic/meson,ernestask/meson,trhd/meson,mesonbuild/meson,mesonbuild/meson,becm/meson,mesonbuild/meson,ernestask/meson,jpakkane/meson,thiblahute/meson,trhd/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,thiblahute/meson,jpakkane/meson,becm/meson,mesonbuild/meson,mesonbuild/meson,jeandet/meson,jeandet/meson,jpakkane/meson,trhd/meson,thiblahute/meson,becm/meson,MathieuDuponchelle/meson,trhd/meson,ernestask/meson,pexip/meson,trhd/meson,jeandet/meson,pexip/meson,thiblahute/meson,mesonbuild/meson,QuLogic/meson,jeandet/meson,trhd/meson,QuLogic/meson,jeandet/meson,MathieuDuponchelle/meson,pexip/meson,pexip/meson,thiblahute/meson